1. Field of the Invention
The present invention relates to a sheet stacking apparatus configured to be able to align a sheet stacked on a stacking tray, an image forming apparatus, and a sheet processing apparatus.
2. Description of the Related Art
Hitherto, there is known a sheet stacking apparatus adapted to be able to align sheets discharged out to a stacking tray for stacking sheets in a sheet widthwise direction (referred to simply as a “width direction” or “widthwise” hereinafter) orthogonal to a direction in which the sheet is discharged (referred to simply as a “discharge direction” hereinafter) as disclosed in Japanese Patent Application Laid-open No. 2002-211829 for example.
The sheet stacking apparatus described in Japanese Patent Application Laid-open No. 2002-211829 includes a pair of aligning members capable of moving in the width direction above a stacking tray. When a sheet is discharged out to the stacking tray, the sheet stacking apparatus aligns the sheet widthwise by moving the pair of aligning members in the width direction so that the aligning members come in contact with both widthwise ends of the sheet.
By the way, the sheet stacking apparatus described in Japanese Patent Application Laid-open No. 2002-211829 aligns a sheet by sandwiching the sheet widthwise between the pair of aligning members, i.e., by making the pair of aligning members come into contact with the both widthwise ends of the sheet, by moving the aligning members widthwise after the elapse of a predetermined time since when the sheet has been discharged out of a sheet discharging portion. The predetermined time until when the pair of aligning members starts to move is set based on a time until when the sheet discharged on an inclined stacking surface of the stacking tray abuts an abutting member against which an end in the discharge direction of the sheet is to be abutted by its own weight or the like. That is, the sheet stacking apparatus is configured so that normally the sheet stacking apparatus aligns the sheet in the width direction by the pair of aligning members after the elapse of the predetermined time since when the sheet has been aligned in the discharge direction by abutting against the abutting member.
However, if such move of the sheet is delayed by some reason, there is a possibility that the predetermined time elapses before the sheet reaches the abutting member and the sheet moving on the inclined stacking tray is held by the pair of aligning members before reaching the abutting member. If the sheet is held and aligned by the pair of aligning members here, there is also a possibility that the sheet in the move is stopped. When there is no sheet on the stacking tray in particular, the sheet is less movable on the stacking surface than a case in which another sheet is already stacked on the stacking tray, because level of kinetic friction force between the stacking surface and the sheet is higher than that between sheets. Due to that, if the sheet is held in this condition from the both ends in the width direction by the pair of aligning members, there is such a possibility that the sheet is stopped before abutting the abutting member. If the sheet is stopped during its move, it becomes even more difficult to move the sheet again because of a static friction force which is caused between a halted sheet and the stacking surface of the stacking tray and which is greater than the kinetic friction force between the stacking surface and a sheet. Due to that, it is unable to align the sheet in the discharge direction by the abutting member, possibly causing stacking misalignment in the discharge direction among the sheets discharged first and discharged afterward. In short, the sheet stacking apparatus has the possibility of causing the stacking misalignment in the discharge direction among sheets discharged first and thereafter if the sheet moving on the inclined stacking surface is stopped on its way by being held by the pair of aligning members.